Fix possible crashing due to tt race in legal.

pseudo_legal does not handle cases where there is only one checker and the move in tt don't comply with the pattern generated by movegen. Causing legal() failed to work with needSlowCheck off.

Crash report by @skystarspython

src/position.cpp

@@ -183,8 +183,8 @@ void Position::set_check_info() const {
     Square ksq = king_square(~sideToMove);
 
     // We have to take special cares about the cannon and checks
-    st->needSlowCheck = more_than_one(checkers())
-                     || (attacks_bb<ROOK>(king_square(sideToMove)) & pieces(~sideToMove, CANNON));
+    st->needSlowCheck =
+      checkers() || (attacks_bb<ROOK>(king_square(sideToMove)) & pieces(~sideToMove, CANNON));
 
     st->checkSquares[PAWN]   = pawn_attacks_to_bb(sideToMove, ksq);
     st->checkSquares[KNIGHT] = attacks_bb<KNIGHT_TO>(ksq, pieces());